Study links radiation exposure, thyroid cancer

A team of American and Russian researchers has established a scientific link between radiation released in the 1986 Chernobyl disaster and escalating thyroid cancer in people exposed as children when the Soviet reactor exploded, killing 30 and threatening the health of five million people.

In their study published this month in Radiation Research, the scientists report an incidence of thyroid cancer 45 times greater among those who got the highest radiation dose from Chernobyl compared with the lowest-dose group. The April 1986 explosion is considered the world’s worst nuclear accident.

It’s the first study to establish a dose-response relationship from Chernobyl, said Scott Davis, a Ph.D. biostatistician at the Fred Hutchinson Cancer Research Center in Seattle and the study’s co-investigator. The study was funded by the U.S. Office of Naval Research.

Based on his Chernobyl work, Davis was elected earlier this year to the Russian Academy of Medical Sciences – the first foreign epidemiologist to be honored by the scientific group.

“Prior to Chernobyl, thyroid cancer in children was practically nonexistent. Today we see dozens and dozens of cases a year in the regions contaminated by the disaster, and the incidence continues to rise,” Davis said.

The study may help officials better predict long-term health effects in the event of a nuclear accident or terrorist attack and will also help public health officials deal with the aftermath of Chernobyl, he added.

A similar 1999 Fred Hutchinson study failed to find a link between Hanford’s chronic releases of radioactive iodine-131 from plutonium production during World War II and the Cold War and thyroid cancer in 3,440 people exposed as children. That $19.5 million study, conducted for the U.S. government, was criticized by the National Academy of Sciences for its weak statistical power – its ability to detect a radiation effect.

The Chernobyl study team focused on people in Bryansk, a region located 66 miles northeast of Chernobyl that was the most heavily contaminated.

Using a local cancer registry, the researchers identified 26 people with thyroid cancer who were under 20 when the reactor blew up. They also identified 52 healthy people from the general population for comparisons. The study participants were interviewed at home by Russian doctors, who asked them where they were during the accident and about their diet and lifestyle.

Individual doses are dependent on uptake of food contaminated with iodine-131, a radioactive element that is absorbed in the thyroid gland. The main source of the human contamination was drinking milk from cows that grazed on contaminated grass, the researchers said.

Because the short-lived iodine-131 had already decayed away, the scientists estimated its presence by measuring longer-lived cesium-137 deposited in the soil after the reactor accident. Cesium-137 takes 30 years to lose half its radiation.

The study’s limitations include its small sample size. Efforts are under way to study a larger population, including older Chernobyl survivors.

It took several years after the 1986 accident for the Fred Hutchinson scientists to get permission to work in Russia. In 1990, a Russian helicopter pilot who was among those trying to contain the Chernobyl radiation developed leukemia and came to Fred Hutchinson for a bone-marrow transplant. After his treatment, an informal exchange began between the Seattle scientists and the National Center for Hematology in Moscow.

In 1992, after the collapse of the Soviet Union, a research consortium was formed to study the long-term health effects of Chernobyl. Davis made 30 trips, walking the grounds of the defunct reactor and surveying the 30-kilometer evacuation zone.

By the early 1990s, new cases of thyroid disease in Belarus and Ukraine, many in young children, continued to increase. There are now hundreds of cases of the disease in children in the three counties most heavily contaminated by the accident, the study found.

Besides the Chernobyl and Hanford children, other radiation-exposed civilians who’ve been studied include Japanese atomic bomb survivors at Hiroshima and Nagasaki; residents of the Marshall Islands exposed to hydrogen bomb tests and Utah schoolchildren showered with fallout from nuclear bomb tests in Nevada.

The Utah study reported a slight excess of thyroid cancers associated with radiation exposure, while the Marshall Islands study found an increase in benign and malignant thyroid nodules among residents of the Rongelap and Utirik atolls.